1. Field of the Invention
The invention provides a process for making flexible circuits comprising the step of etching polymer films by masking portions thereof with crosslinked negative aqueous processible photoresists and dissolving unmasked portions thereof with concentrated aqueous base.
2. Description of the Related Art
A typical step in the fabrication of electronic circuitry on polymeric carrier films is to etch through the polymeric film to allow for subsequent connection through the backside to other devices or, where applicable, connection of frontside and backside circuitry. The selective etching or milling is necessary to allow electrical connections to be made between the differing conductive layers. Those portions of the polymeric film for which etching is not desired must be masked with a protective coating. The masking process can be satisfactorily done with the art of photolithography whereby an etch-insensitive material is applied and patterned using normal lithographic methods, i.e., exposure and development.
Solvent processible, water-insoluble or water-insensitive photoresists are ideal for masking where the subsequent etching process is aqueous based. Solvent processible resists are both developed and stripped using solvents and remain intact during such a process and thus protect any covered areas of the polymeric film while allowing etching of areas not protected by resists. However, solvent processible resists are less desirable from environmental and economic perspectives, and thus aqueous processible resists are preferred.
However, aqueous processible resists are stripped using dilute bases. Therefore, they have not been used as masks in processes involving aqueous base chemical etching of polymers.
U.S. Pat. No. 4,039,371 discloses a composition for etching polyimide based polymers comprising a tetraalkyl ammonium hydroxide and acetic acid, tartaric acid or oxalic acid. The resists disclosed to be useful as masks are positive resists; those resists specifically mentioned are all solvent processible and coated by spincoating techniques.
U.S. Pat. No. 3,871,930 discloses the use of a commercial solvent processible photomask for use with an etching solution comprising a suitable base in a nonaqueous solvent such as dimethylsulfoxide.
U.S. Pat. No. 3,395,057 discloses the use of solvent processible photoresists and copper foils to mask those areas not to be etched.
U.S. Pat. No. 4,857,143 discloses a process for etching a fully or substantially fully cured polyimide layer which comprises contacting such layer with an aqueous solution at least about 3 molar of a metal hydroxide and at least about 0.5 molar of a metal carbonate, metal sulfate or metal phosphate. It is specifically disclosed that copper is used as the etching mask for the polyimide film.
It has now been discovered that the etching step may be done with hot, concentrated, e.g., 35% potassium hydroxide, using a crosslinked aqueous processible photoresist as a mask.
Surprisingly, the crosslinked photoresist protects the masked area such that etching of the polymer film is completed only where desired without significantly swelling or delaminating the photoresist. This is especially surprising when it is considered that the aqueous photoresists will delaminate from the film when exposed to dilute basic solutions, e.g., 5% or less. This process avoids use of a solvent processible resist, while also eliminating processing steps and the need for copper masks.